Molecular Beam Epitaxy Research Articles

Gated InAs quantum dots embedded in surface acoustic wave cavities for low-noise optomechanics

Self-assembled InAs quantum dots (QDs) are promising optomechanical elements due to their excellent photonic properties and sensitivity to local strain fields. Microwave-frequency modulation of photons scattered from these efficient quantum emitters has been recently demonstrated using surface acoustic wave (SAW) cavities. However, for optimal performance, a gate structure is required to deterministically control the charge state and reduce the charge noise of the QDs. Here, we integrate gated QDs and SAW cavities using molecular beam epitaxy and nanofabrication. We demonstrate that with careful design of the substrate layer structure, integration of the two systems can be accomplished while retaining the optimal performance of each subsystem. These results mark a critical step toward efficient and low-noise optomechanical systems that truly leverage the excellent properties of semiconductor QDs.

Selective area epitaxy of in-plane HgTe nanostructures on CdTe(001) substrate

Semiconductor nanowires (NWs) are believed to play a crucial role for future applications in electronics, spintronics and quantum technologies. A potential candidate is HgTe but its sensitivity to nanofabrication processes restrain its development. A way to circumvent this obstacle is the selective area growth technique. Here, in-plane HgTe nanostructures are grown thanks to selective area molecular beam epitaxy on a semi-insulating CdTe substrate covered with a patterned SiO2mask. The shape of these nanostructures is defined by the in-plane orientation of the mask aperture along the <110>, <11¯0>, or <100> direction, the deposited thickness, and the growth temperature (GT). Several micron long in-plane NWs can be achieved as well as more complex nanostructures such as networks, diamond structures or rings. A good selectivity is achieved with very little parasitic growth on the mask even for a GT as low as 140 °C and growth rate up to 0.5 monolayer per second. For <110> oriented NWs, the center of the nanostructure exhibits a trapezoidal shape with {111}B facets and two grains on the sides, while <11¯0> oriented NWs show {111}A facets with adatoms accumulation on the sides of the top surface. Transmission electron microscopy observations reveal a continuous epitaxial relation between the CdTe substrate and the HgTe NW. Measurements of the resistance with four-point scanning tunneling microscopy indicates a good electrical homogeneity along the main NW axis and a thermally activated transport. This growth method paves the way toward the fabrication of complex HgTe-based nanostructures for electronic transport measurements.

Arsenic-doped HgCdTe: FTIR photoluminescence and photoreflectance spectroscopy study

Photoluminescence (PL) and photoreflectance (PR) spectroscopy were used for the optical study of arsenic doping of HgCdTe grown by molecular beam epitaxy (MBE). Un-doped and arsenic-doped material with cadmium telluride molar fraction x = 0.29 grown under similar conditions and subjected to similar types of annealing was studied. The PL spectra of the un-doped material featured signatures of intrinsic defects associated with mercury vacancy, excessive tellurium, and related complexes. In the arsenic-doped material, optical signatures of these defects appeared to be suppressed. After arsenic activation, shallow (7–8 meV) acceptor levels were found in the material. These were attributed to the dopant activation, which was confirmed with electrical studies showing p-type conductivity with hole concentration ∼1016 cm−3. The studies showed that the actual pattern of arsenic doping in HgCdTe can be indeed screened by intrinsic defects, which are inherent to MBE-grown HgCdTe and tend to interact with the dopant.

Realization of a 2D Lieb Lattice in a Metal-Inorganic Framework with Partial Flat Bands and Topological Edge States.

Flat bands and Dirac cones in materials are the source of the exotic electronic and topological properties. The Lieb lattice is expected to host these electronic structures, arising from quantum destructive interference. Nevertheless, the experimental realization of a 2D Lieb lattice remained challenging to date due to its intrinsic structural instability. After computationally designing a Platinum-Phosphorus (Pt-P) Lieb lattice, it has successfully overcome its structural instability and synthesized on a gold substrate via molecular beam epitaxy. Low-temperature scanning tunneling microscopy and spectroscopy verify the Lieb lattice's morphology and electronic flat bands. Furthermore, topological Dirac edge states stemming from pronounced spin-orbit coupling induced by heavy Pt atoms are predicted. These findings convincingly open perspectives for creating metal-inorganic framework-based atomic lattices, offering prospects for strongly correlated phases interplayed with topology.

Growth and electronic structure of the nodal line semimetal in monolayer Cu2Si on Cu(111)

Cu2Si, a single-layer two-dimensional material with a honeycomb structure, has been proposed to have Dirac nodal line fermions. In this study, the synchrotron radiation X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and angle-resolved photoemission spectroscopy (SR-XPS, SR-UPS, and SR-ARPES) techniques were used to investigate the dynamic process of in situ deposition of single-layer Cu2Si on a Cu(111) crystal surface via molecular beam epitaxy (MBE). Cu2Si existed as a monolayer (ML) alloy, and there were competing mechanisms of distinct chemical states of silicon in different growth periods, according to a detailed examination of the experimental SR-XPS and SR-UPS spectra. Additionally, a weak interaction between the Cu2Si ML and Cu(111) was demonstrated via SR-ARPES and first-principles computations. The unique electronic structure of the Cu2Si ML was not destroyed by either this weak interaction or the disordered silicon produced on the surface during the growth process. The study of the Cu2Si growth kinetics provides a guarantee and a basis for the future exploration of the exotic properties of Cu2Si.

Detection of an unintentional Si doping gradient in site-controlled GaN nanowires grown using a Si3N4 mask by spatially resolved cathodoluminescence and Raman spectroscopy

Highly uniform arrays of site-controlled GaN nanowires are synthesized by selective area growth using a Si3N4 mask and molecular beam epitaxy. Systematic modulation of the emission along the nanowire axis is observed in spectrally resolved cathodoluminescence linescans. We show that this intensity change is an indicator of unintentional Si incorporation during growth resulting from the interaction between the impinging Ga atoms and the mask material. The gradual reduction of the cathodoluminescence intensity along the nanowire highlights the important role of the growth geometry within the synthesis reactor, with shadowing from the elongating nanowires inhibiting the reaction with the mask. This gradient in Si doping is confirmed by the quenching of the longitudinal optical phonon line measured in Raman spectra along the nanowire axis. The corresponding carrier density is derived from the frequency of the coupled phonon–plasmon mode. The spectroscopic identification of inversion domain boundaries in the majority of the nanowires is also attributed to the Si incorporation. From temperature dependent cathodoluminescence experiments, we derive the activation energy for excitons bound to these defects.

Silicon nitride shadowed selective area growth of low defect density vertical GaN mesas via plasma-assisted molecular beam epitaxy

Ion bombardment during inductively coupled plasma reactive-ion etching and ion-implantation introduces irreparable crystalline damage to gallium nitride (GaN) power devices, leading to early breakdown and high leakage current. To circumvent this, a bi-layer selective area growth mask was engineered to grow up to 3.0 µm thick epitaxy of GaN using plasma-assisted molecular beam epitaxy as an ion-damage-free alternative to standard epitaxial processing routes. The masks and regrown architectures are characterized via SEM, conductive-atomic force microscopy (AFM), x-ray photo electron spectroscopy, Raman, and cathodoluminescence. Mask deposition conditions were varied to modulate and minimize the stress induced during thermal cycling. The resulting mesas exhibit low leakage, attributed to naturally terminated sidewalls as measured by an innovative perpendicular AFM measurement of the regrown sidewall. The regrown sidewall exhibited RMS (root mean square) roughness of 1.50 (±0.34) nm and defect density of 1.36 × 106 (±1.11 × 106) cm−2. This work provides a method to eliminate defect-inducing steps from GaN vertical epitaxial processing and stands to enhance GaN as a material platform for high-efficiency power devices.

Phase-selective growth of κ- vs β-Ga2O3 and (InxGa1−x)2O3 by In-mediated metal exchange catalysis in plasma-assisted molecular beam epitaxy

Its large intrinsic polarization makes the metastable κ-Ga2O3 polymorph appealing for multiple applications, and the In-incorporation into both κ and β-Ga2O3 allows us to engineer their bandgap on the low-end side. In this work, we provide practical guidelines to grow thin films of single phase κ-, β-Ga2O3 as well as their (InxGa1−x)2O3 alloys up to x = 0.14 and x = 0.17, respectively, using In-mediated metal exchange catalysis in plasma-assisted molecular beam epitaxy (MEXCAT-MBE). The role of substrate temperature, oxidizing power, growth rate, and choice of substrate on phase formation and In-incorporation is investigated. As a result, the κ phase can be stabilized in a narrow deposition window irrespective of the choice of substrate [(i) α-Al2O3 (0001), (ii) 20 nm of (2̄01) β-Ga2O3 on α-Al2O3 (0001), and (iii) (2̄01) β-Ga2O3 single crystal]. Low growth rates/metal fluxes as well as growth temperatures above 700 °C tend to stabilize the β-phase independently. Lower growth temperatures and/or O-richer deposition atmospheres allow to increase the In-incorporation in both polymorphs. Finally, we also demonstrate the possibility to grow (2̄01) β-Ga2O3 on top of α-Al2O3 (0001) at temperatures at least 100 °C above those achievable with conventional non-catalyzed MBE, opening the road for better crystal quality in heteroepitaxy.

Epitaxy of GaSe Coupled to Graphene: From In Situ Band Engineering to Photon Sensing.

2D semiconductors can drive advances in quantum science and technologies. However, they should be free of any contamination; also, the crystallographic ordering and coupling of adjacent layers and their electronic properties should be well-controlled, tunable, and scalable. Here, these challenges are addressed by a new approach, which combines molecular beam epitaxy and in situ band engineering in ultra-high vacuum of semiconducting gallium selenide (GaSe) on graphene. In situ studies by electron diffraction, scanning probe microscopy, and angle-resolved photoelectron spectroscopy reveal that atomically-thin layers of GaSe align in the layer plane with the underlying lattice of graphene. The GaSe/graphene heterostructure, referred to as 2semgraphene, features a centrosymmetric (group symmetry D3d) polymorph of GaSe, a charge dipole at the GaSe/graphene interface, and a band structure tunable by the layer thickness. The newly-developed, scalable 2semgraphene is used in optical sensors that exploit the photoactive GaSe layer and the built-in potential at its interface with the graphene channel. This proof of concept has the potential for further advances and device architectures that exploit 2semgraphene as a functional building block.

Unraveling the electronic structure and magnetic transition evolution across monolayer, bilayer, and multilayer ferromagnetic Fe3GeTe2

Two-dimensional (2D) van der Waals (vdW) magnets have sparked widespread attention due to their potential in spintronic applications as well as in fundamental physics. Ferromagnetic vdW compound Fe3GeTe2 (FGT) and its Ga variants have garnered significant interest due to their itinerant magnetism, correlated states, and high magnetic transition temperature. Experimental studies have demonstrated the tunability of FGT’s Curie temperature, TC, through adjustments in quintuple layer numbers (QL) and carrier concentrations, n. However, the underlying mechanism remains elusive. In this study, we employ molecular beam epitaxy (MBE) to synthesize 2D FGT films down to 1 QL with precise layer control, facilitating an exploration of the band structure and the evolution of itinerant carrier density. Angle-resolved photoemission spectroscopy (ARPES) reveals significant band structure changes at the ultra-thin limit, while first-principles calculations elucidate the band evolution from 1 QL to bulk, largely governed by interlayer coupling. Additionally, we find that n is intrinsically linked to the number of QL and temperature, with a critical value triggering the magnetic phase transition. Our findings underscore the pivotal role of band structure and itinerant electrons in governing magnetic phase transitions in such 2D vdW magnetic materials.

The optical properties of 3 + 3 macrocyclic Schiff base thin material obtained by the Molecular Beam Epitaxy method

3 + 3 optically active macrocyclic Schiff bases were synthesized in the reaction between 4-tert-butyl-2,6-diformylphenol with (1R,2R)-(+)-1,2-diphenylethylenediamine (S1) or (1S,2S)-(−)-1,2-diphenylethylenediamine (S1a). The new compounds were spectroscopically characterised by NMR, IR, X-ray (S1a), UV–Vis and fluorescence spectroscopy. The S1a molecule creates channels with distances between oxygen atoms ranging from 5.8-6.3 Å and sufficiently large to host acetonitrile molecule. Both compounds exhibit green-yellow emission in solution and solid state. Thin layers of the S1 compound obtained via Molecular Beam Epitaxy (MBE) were characterised by scanning electron microscopy with energy-dispersive X-ray spectroscopy SEM/EDS and atomic force microscopy (AFM). The optical properties of the S1/Si thin material were analysed using spectroscopic ellipsometry (SE), fluorescence spectroscopy and synchrotron radiation (SR). The time constant for the decay investigated under SR, denoted by τ1, was determined to be approximately 1.02 ns, suggesting a fast deactivation process of the excited electronic state in the S1/Si material. The ellipsometric analysis of the S1/Si layer showed semiconducting behaviour with pronounced absorption features in the UV range, attributed to π → π* and n → π* transitions, characteristic of Schiff bases. The band-gap energy, determined using the Tauc method, is 3.46 ± 0.01 eV. These analyses highlight the material’s potential in applications requiring precise control of optical properties. In the emission spectrum of S1a, a significant emission peak of approximately 561 nm indicates the presence of a prominent emissive process within this wavelength. The S1a compound is emissive in the yellow-green region of the spectrum and has a longer decay time, which suggests that it can be used in sensing optical technologies.

Designing TaC Virtual Substrates for Vertical AlxGa1−xN Power Electronics Devices

Power electronics are critical for a sustainable energy future, playing a key role in electrification and integration of renewable energy sources into the grid. Advances in ultrawide band gap materials are needed to handle higher powers in smaller form factors while reducing electrical and thermal losses. High Al content AlxGa1−xN is theoretically capable of meeting these demands, but its impact in power electronics has been severely restricted by a lack of substrates that can satisfy conductivity, lattice matching, and/or thermal expansion requirements. We demonstrate that electrically conductive TaC can be used as a virtual substrate for AlxGa1−xN heteroepitaxy. Scaleably sputtered TaC grown on Al2O3, followed by high-temperature face-to-face annealing, produces a thin film TaC template with an effective hexagonal lattice constant matched to Al0.70Ga0.30N. Annealing of the TaC promotes recrystallization, significantly improving crystallinity and reducing crystalline defects from as-deposited columnar grains to a step-and-terrace surface morphology, enabling the subsequent growth of high-quality Al0.70Ga0.30N by molecular beam epitaxy. X-ray diffraction and scanning transmission electron microscopy confirm that the AlxGa1−xN layer is heteroepitaxially aligned, strain-free, and lattice-matched, transitioning abruptly from TaC to AlxGa1−xN without intermediate phases. These results demonstrate TaC virtual substrates as electrically conductive, lattice-matched, and thermally compatible templates for vertical AlxGa1−xN devices that can meet the growing power needs of a sustainable energy future. Published by the American Physical Society 2024

Light Emission of Sn/Ge MQW pin Diodes on Si

Sn/Ge multi-quantum well diodes integrated on Si were grown pseudomorphically on Ge virtual substrates on Si using molecular beam epitaxy. Temperature-dependent electroluminescence analyses were conducted on these devices across a range of temperatures, from 290 K to 12 K. This analysis identified four distinct energy transitions across the entire temperature range. Two of the peaks were observed at high temperatures and were no longer discernible as the temperature decreased. The remaining two peaks were only measured at low temperatures. By analyzing them with a Ge reference diode, one of the peaks was identified as a defect peak. The intensity of the other peak increased as the temperature decreased, which is consistent with the behavior of a direct transition in an indirect semiconductor material.

Epitaxial Growth of Large-Scale α-Phase Antimonene.

Two-dimensional materials composed of elements from the 15th group of the periodic table remain largely unexplored. The primary challenge in advancing this research is the lack of large-scale layers that would facilitate extensive studies using laterally averaging techniques and enable functionalization for the fabrication of novel electronic, optoelectronic, and spintronic devices. In this report, we present a method for synthesizing large-scale antimonene layers, on the order of cm2. By employing molecular beam epitaxy, we successfully grow a monolayer film of α-phase antimonene on a W(110) surface passivated with a single-atom-thick layer of Sb atoms. The formation of α phase antimonene is confirmed through scanning tunneling microscopy and low-energy electron diffraction measurements. The isolated nature of the α-phase is further evidenced in the electronic structure, with linearly dispersed bands observed through angle-resolved photoelectron spectroscopy and supported by ab initio calculations.

Arsenic-flux dependence of surface morphology in InAs homoepitaxy

Surface morphology in molecular-beam epitaxy of InAs(001), (111)A, and (111)B has been studied using scanning tunneling microscopy. The surface morphologies of InAs strongly depend on substrate temperature, substrate orientation, and As/In flux ratio. The size and density of two-dimensional InAs islands on the (001) surface decreases and increases, respectively, as the As/In flux ratio is increased. On the other hand, island size (density) is increased (decreased) with the increase in As flux on (111)A and (111)B surfaces. Surface reconstructions on growing surfaces strongly affect the diffusion and incorporation kinetics of In atoms, thereby resulting in the observed surface morphologies.

A high-order energy stable method for the MBE models with slope selection by using Lagrange multiplier approach

In this work, we develop high-order convex splitting implicit–explicit Runge–Kutta methods for Molecular Beam Epitaxy (MBE) model with slope selection, which plays key roles in materials science and physics for describing various phenomena, such as phase transitions, interactions and interfacial dynamics. Since the epitaxy surface height evolution equation is viewed as a dynamical form of a L2-gradient flow, MBE has the highly similar growing processes as the growing facets in phase-ordering process in magnetic systems. Within this context, we focus our attention on the systems with multiple components possess greater physical significance than their classical (single-component) counterparts. We rigorously prove that the proposed schemes both preserve the energy dissipation and mass conservation. Finally, the accuracy and efficiency of proposed schemes are demonstrated by some numerical experiments.

Molecular Beam Epitaxy of Homogeneous Topological HgTe on Doped InAs Substrate

AbstractHgTe has been considered to be one of the most versatile topological materials. Depending on the in‐plane strain, Weyl and Dirac semi‐metal, as well as topological insulator phases, are feasible. Here, for the first time it is reported, that an InAs:S substrate promotes an initial 2D growth of a ZnTe thin layer and subsequent high crystalline quality ZnTe/CdTe superlattices serve as a smooth and continuous virtual substrate for the growth of (Cd,Hg)Te/HgTe/(Cd,Hg)Te heterostructures with unstrained quantum well HgTe (topological insulator) and compressively strained bulk HgTe (Weyl semimetal) by molecular beam epitaxy. Compared with the superlattices previously grown on GaAs substrates, the quantum well and bulk heterostructures exhibit homogeneous surfaces with root mean square roughnesses of 0.88–0.93 nm, which is three times lower than those observed for 3D islands (2.7–3.4 nm) on GaAs substrates. Additionally, magnetotransport measurements confirm high electronic quality and demonstrate that the S‐doped InAs substrate can be used as an effective back gate. These results manifest a (big) step forward toward the improvement of micro‐ and nanometer‐sized top‐ and back‐gated device fabrication on topological materials.

Transport and electronic structure properties of MBE grown Sn doped Ga2O3 homo-epitaxial films

In this work, we report the transport, defect state and electronic structure properties of unintentionally doped (UID) and Sn doped β-Ga2O3 homo-epitaxial thin films grown by molecular beam epitaxy (MBE) with electron density ranging from 2.1 × 1016 to 2.8 × 1019 cm−3. The UID film with an electron density of 2.1 × 1016 cm−3 exhibits a notable RT mobility of 129 cm2/Vs and a peak mobility of 900 cm2/Vs at 80 K, achieving the state-of-the-art level for MBE-grown Ga2O3 films. Temperature dependent Hall measurement reveal that Sn dopants have an activation energy of 56.7 meV. Synchrotron-based photoemission spectroscopy were further used to study insights into the evolution of electronic properties induced by Sn doping. An in-gap defect state was observed at the 1.5 eV above the valence band maximum for the Sn-doped Ga2O3 film. The in-gap state acts as self-compensating centers affecting the overall doping efficiency and mobility. Furthermore, photoemission spectroscopic study also reveals an upward surface band bending existing at the surface region of Sn doped Ga2O3 films. The identification of the in-gap state and surface upward band bending have significant implications for understanding the doping mechanisms in Ga2O3 and its electronic device applications.

Plasmonic-Metal Nanostructures-Enhanced Photoconductive Terahertz Emission and Detection

Photoconductive antennas based on low-temperature gallium arsenide (LT-GaAs) with a gold plasmonic-gold grating made of nanorods and nanoislands have been developed and fabricated. The antennas were produced using molecular beam epitaxy and electron-beam nanolithography. LT-GaAs samples with a high annealing temperature of 943 K were employed in the fabrication process. It has been demonstrated that plasmonic nanostructures significantly enhance the efficiency of light-to-terahertz radiation conversion by photoconductive antennas.

Determining the oxidation stability of SnSe under atmospheric exposure

AbstractUnderstanding surface stability becomes critical as 2D materials like SnSe are developed for piezoelectric and optical applications. SnSe thin films deposited by molecular beam epitaxy showed no structural changes after a two-year exposure to atmosphere, as confirmed by X-ray diffraction and Raman spectroscopy. X-ray photoelectron spectroscopy and reflectivity show a stable 3.5 nm surface oxide layer, indicating a self-arresting oxidative process. Resistivity measurements show an electrical response dominated by SnSe post-exposure. This work shows that SnSe films can be used in ambient conditions with minimal risk of long-term degradation, which is critical for the development of piezoelectric or photovoltaic devices. Graphical Abstract